CN107432072B - Light-emitting diode heat return control device and method - Google Patents

Abstract

The invention discloses a heat return control circuit electrically connected to a light emitting diode LED driver. The heat foldback control circuit includes a voltage divider and a shunt regulator. The voltage divider includes a first resistor element, a second resistor element configured in series with the first resistor element, and an output. The first resistor assembly has a first resistance and the second resistor assembly has a second resistance that varies in response to a temperature at a reference point. The output is configured to output a reference voltage based on the first resistance and the second resistance. The shunt regulator is configured in parallel with the voltage divider and is configured to receive the reference voltage and control a driver output of the LED driver based on the reference voltage.

Description

Light-emitting diode heat return control device and method

Cross-references to related cases

This application claims the benefit of US Provisional Application No. 62/118,746, filed February 20, 2015, which is incorporated herein by reference in its entirety.

Background technique

The present application relates to control devices and methods for lamp fixtures, such as light emitting diode (LED) lamp fixtures.

LEDs are increasingly used in a wide variety of lighting applications, such as automotive headlights and taillights, street lighting, architectural lighting, backlights and flashlights for liquid crystal display devices, to name a few. LEDs have significant advantages over conventional lighting sources such as incandescent and fluorescent lamps. These advantages include high power efficiency, good directionality, color stability, high reliability, long life, small size and environmental safety.

SUMMARY OF THE INVENTION

Identify and discuss some of the challenges related to thermal management and associated with most LEDs and their applications. Some of these thermal challenges can be mitigated or resolved by using a thermal foldback control circuit that provides control signals to the dimmer controls embedded in the LED driver. Next, the components, structures, functions, and implementations of various configurations of the heat foldback control circuit are described.

Although LEDs represent a relatively new market for lighting applications, the use of LEDs as a replacement for conventional lighting products comes with certain thermal challenges that are difficult to solve. That is, the efficiency of an LED is largely dependent on the junction temperature of the device. For example, the lumens (or light intensity) produced by an LED typically decreases linearly with increasing junction temperature. The lifetime of the LED also decreases as the junction temperature increases.

Some lighting system manufacturers address these thermal challenges by designing systems with appropriate heat sinks, high thermal conductivity enclosures, and other thermal design techniques. However, these thermal design techniques do not treat the LED driver integrated circuit (IC) as a control component in a thermal management system.

The LED driver can be used as a control component to modify the drive current of the LED based on temperature. Therefore, using an LED driver with intelligent over temperature protection can provide an additional control mechanism that can significantly increase the life of the LED light source, ensure rated life, and reduce the incidence of defective products.

Depending on the lighting manufacturer and application, the useful life of LED lighting products ranges from approximately 20,000 hours to greater than 50,000 hours, compared to less than 2,000 hours for incandescent light bulbs. However, as the junction temperature increases, not only does the light output of the LED decrease, but also the lifetime of the LED. Smart thermal protection can also help reduce system losses by enabling system integrators to design heat sinks with low safety margins.

Generally, the design of the thermal management system of the LED lighting device focuses on the design of the heat sink and the printed circuit board (PCB), but does not consider the possibility of thermal management by the LED driver IC and the driver circuit. Intelligent over-temperature protection by the LED driver IC can significantly increase the lifetime of the LED light source.

Temperature protection with LED driver ICs has been implemented in various ways. Some LED driver devices include sense pins to which an external temperature sensor can be attached. Different temperature sensing devices including diodes, on-chip sensors, positive temperature coefficient (PTC) or negative temperature coefficient (NTC) thermistors can be used in LED lighting applications to help protect the LEDs from overheating. After the temperature is accurately sensed, a response to any over-temperature conditions is then implemented. One response is to rapidly turn off the drive current to the LED when the threshold temperature is exceeded. Lighting fixtures that incorporate this type of response then "restart" the light source when the temperature drops, or alternatively, the lighting fixture waits until a power cycle occurs, which typically restarts the light. However, there are disadvantages associated with this approach.

For example, abrupt shutdown methods often require that the threshold temperature be set very high to avoid improperly triggering lamp shutdown. While this high threshold protects the lamp from destructive failure, it can still result in a significant reduction in the lifetime of the LED. Furthermore, turning off the LED current means that the lamp is suddenly turned off. This can lead to serious conditions like panic in public areas. Many known LED drivers automatically restart after the system has cooled, and once restarted the system repeatedly heats up and shuts down, resulting in a disturbing "flickering" effect.

Embodiments of the present application help address the above-mentioned problems by, in one embodiment, providing a thermal foldback control circuit that is electrically connected to a light emitting diode (LED) driver. The thermal foldback control circuit includes a voltage divider and a shunt regulator. The voltage divider includes a first resistor element, a second resistor element configured in series with the first resistor element, and an output. The first resistor assembly has a first resistance and the second resistor assembly has a second resistance that varies in response to temperature at the reference point. The output is configured to output a reference voltage based on the first resistance and the second resistance. The shunt regulator is configured in parallel with the voltage divider and is configured to receive a reference voltage and control the driver output of the LED driver based on the reference voltage.

In another embodiment, the present application provides a light emitting diode (LED) system comprising: one or more LEDs; an LED driver that provides power to the one or more LEDs; and a thermal foldback control circuit. The heat foldback control circuit is electrically connected to the LED driver and is configured to output a control signal to the driver based on a temperature at a reference point.

In another embodiment, the present application provides a method of controlling power to one or more LEDs. The method includes: sensing a temperature at a reference point; comparing the sensed temperature to a predetermined temperature threshold; and reducing the flow to the temperature when the sensed temperature exceeds the predetermined temperature threshold Power to one or more LEDs.

Other aspects of the application will become apparent upon consideration of the detailed description and accompanying drawings.

Description of drawings

FIG. 1 is a light emitting diode (LED) system according to an embodiment of the present application.

2 is a heat foldback control circuit of the LED system of FIG. 1 according to an embodiment of the present application.

3 is a heat foldback control circuit of the LED system of FIG. 1 according to an embodiment of the present application.

4A is a graph illustrating the relationship between sensed temperature and percent output current of the LED driver of the LED system of FIG. 1 according to an embodiment of the present application.

4B is a graph illustrating the relationship between the sensed temperature and the output voltage of the heat foldback control circuit of the LED system of FIG. 1 according to an embodiment of the present application.

5 is a perspective view of a thermal foldback device connected to an LED driver of the LED system of FIG. 1 according to an embodiment of the present application.

FIG. 6 is a top view of the heat-reflecting device of FIG. 5 according to an embodiment of the present application.

7 is a side view of the heat refrigerating device of FIG. 5 according to an embodiment of the present application.

8 is a side view of the heat refrigerating device of FIG. 5 according to an embodiment of the present application.

9 is a flow chart illustrating the operation of the heat foldback device of the LED system of FIG. 1 according to an embodiment of the present application.

10 is a flowchart illustrating the operation of the heat foldback device of the LED system of FIG. 1 according to an embodiment of the present application.

Detailed ways

Before explaining any embodiment of the present application in detail, it is to be understood that the present application is not limited in its application to the details of construction and arrangement of components set forth in the following description or illustrated in the following drawings. The application is capable of other embodiments and of being practiced or carried out in various ways.

It should be noted that the phrase "series-type configuration" as used herein refers to a circuit arrangement in which the described elements are generally arranged in a sequential manner such that the output of one element is coupled to the input of another element, but the same current may not necessarily flow through each a component. For example, in a "series-type configuration," additional circuit elements may be connected in parallel with one or more of the elements in the "series-type configuration." Furthermore, additional circuit elements may be connected at nodes in a series-type configuration such that there are branches in the circuit. Therefore, elements in a series configuration do not necessarily form a true "series circuit".

Additionally, the phrase "parallel configuration" as used herein refers to a circuit arrangement in which the described elements are generally arranged in such a way that one element is connected to another element such that the circuit forms a parallel branch of the circuit arrangement. In this configuration, the elements may not necessarily individually have the same potential difference across the individual elements of the circuit. For example, in a parallel-type circuit configuration, it is possible for two circuit elements connected in parallel with each other to be connected in series with one or more additional elements of the circuit. Thus, a circuit in a "parallel configuration" may include elements that do not necessarily individually form a true parallel circuit.

Figure 1 depicts an embodiment of a system for controlling the temperature of a light source. According to this embodiment, overheating of the LED assembly is attenuated and sudden turn-off of the LEDs is eliminated. Thermal foldback device 100 is connected to LED driver 102, which controls LED engine 104 having one or more light sources (eg, LED modules (not shown)). The LED driver 102 has a power connection 106 and an output connection 108 . In various embodiments, the power connection 106 includes an AC line, an AC neutral, and a ground terminal that can be coupled to an alternating current (AC) power source (eg, commercial grid power). In another embodiment (not shown), the power connection includes a positive direct current (DC) terminal and a negative DC terminal from a DC power source. The LED driver 102 also has output connections 108 including DC positive and DC negative connections to the LED engine 104 . The LED driver generates current and voltage (eg, driver output) to the LED engine 104 to power the LEDs. Although the previous discussion was directed to LEDs, the devices and methods described herein can be modified for use with other light sources, such as fluorescent lamps, when excessive heat generated by the light source can degrade electronic components associated with the production of light , as will be understood by those skilled in the art.

The LED driver 102 includes a dimmer interface 110 designed to connect to a standard dimmer switch (not shown) using a positive electrical connection and a negative electrical connection 112 . In one embodiment, the dimmer interface 110 drives current and senses voltage. The sensed voltage output of the dimmer interface 110 determines the current or voltage produced by the LED driver to the LED. Typically, dimmer switches contain some type of potentiometer to change the resistance, which changes the voltage produced by the dimmer switch. In various embodiments, the dimmer interface 110 is a 0V to 10V dimmer interface that senses voltages between 0 volts (V) and 10 volts. The LED driver 102 has a 0V to 10V dimmer interface 110, eg, a Dialog Semiconductor IW3630, which is commercially available and includes various components to perform various functions, as will be understood by those skilled in the art.

The thermal foldback device 100 may use the dimmer interface 110 of the LED driver 102 for thermal management. The heat foldback device 100 is connected to the LED driver 102 through the dimmer interface 110 . The heat foldback device 100 is designed to sense the temperature of a particular point based on the location of the heat foldback device 100 or, in particular, the thermistor (or resistor) of the heat foldback device 100 . If the sensed temperature exceeds the reference temperature, the heat foldback device 100 automatically provides a signal to the LED driver 102 to dim the light source. The LED driver 102 dims the light module by reducing the current supplied to the light source. The reduced light reduces the heat generated by the light source, thereby preventing any temperature increase and effectively reducing the temperature. If the temperature continues to increase, the thermal foldback device 100 causes the LED driver 102 to further dim the lamp and in due course the driver can be configured to turn off the light source completely. Once the temperature returns to a safe operating level, the heat return device 100 signals the LED driver 102 to increase the current or voltage supplied to the light source back to normal lighting levels. Through this process, the heat return device 100 can be used to set an equilibrium level of LED lighting based on a predetermined maximum allowable temperature indicative of overheating. By preventing overheating, thermal foldback device 100 helps increase the life of LED driver 102 and LED engine 104 and protects these and other components from premature failure.

In various embodiments, the heat refrigerating device 100 is connected to or near a reference point to measure the temperature at a particular location. For example, thermal foldback device 100 may be connected to LED driver 102, LED engine 104, LEDs, or other hot or temperature sensitive points in a lamp fixture. The connection must be thermal and mechanical. In various embodiments, the thermal foldback device 100 is connected to more than one reference point, or multiple thermal foldback devices 100 may be connected to different reference points. When a plurality of thermal foldback devices 100 are used, the thermal foldback devices 100 may be connected in parallel. The upper limit of the monitored reference point depends on the current rating of the dimming driver source, the size and configuration of the associated lamp fixture, as will be understood by those skilled in the art.

FIG. 2 depicts one embodiment of a thermal foldback device 100 implemented as a control circuit 120 . The control circuit 120 is a temperature sensitive module that measures the temperature at the point of interest and provides a signal to the LED driver 102 via the dimming interface 110 . According to one embodiment, the control circuit 120 includes a first resistor element 122 having a first resistance and a second resistor element 124 having a second resistance. In some embodiments, the first resistor assembly 122 and the second resistor assembly 124 are in a series configuration.

The first resistor component 122 may be a resistor or a thermistor, such as, but not limited to, a negative temperature coefficient (NTC) type thermistor or a positive temperature coefficient (PTC) type thermistor. The second resistor component 124 may be a resistor or a thermistor, such as, but not limited to, a negative temperature coefficient (NTC) type thermistor or a positive temperature coefficient (PTC) type thermistor. In one embodiment, at least one resistor assembly 122, 124 is a thermistor. If both resistor assemblies 122, 124 are thermistors, the control circuit 120 of the thermal foldback device 100 may also provide dimming functionality. In one embodiment, the control circuit utilizes a single thermistor, so only one of the first resistor assembly 122 and the second resistor assembly 124 is a thermistor and the other is a resistor.

The control circuit 120 also includes a shunt regulator 126 . In some embodiments, the shunt regulator 126 is in a parallel configuration with the first resistor assembly 122 and the second resistor assembly 124 . In various embodiments, shunt regulator 126 (or shunt voltage regulator) is a low voltage adjustable precision shunt regulator (eg, TLV431). In various embodiments, the shunt regulator 126 utilizes Zener diodes, avalanche breakdown diodes, or voltage regulator tubes. In some embodiments, the shunt regulator 126 is a three-terminal device having anode, cathode, and reference voltage terminals. The anode of the shunt regulator 126 is electrically connected to the first terminal of the second resistor assembly 124 and the negative terminal 128 of the control circuit 120 (or the dimming interface 110). The cathode of the shunt regulator 126 is electrically connected to the first terminal of the first resistor assembly 122 and the positive terminal 129 of the control circuit 120 (or the dimming interface 110). The reference input voltage terminal of the shunt regulator 126 is electrically connected to the first resistor element 122 and the second resistor element 124 (ie, the second terminal of the first resistor element 122 and the second terminal of the second resistor element 124 ) )between. The shunt regulator 126 has a specified thermal stability over an industrial and commercial applicable temperature range. In an embodiment, the control circuit 120 is powered by a current source. The current source may be provided from the current supplied to the light source or the secondary output current from the LED driver 102 (eg, 0V to 10V dimmer interface 110).

The first resistor component 122 and the second resistor component 124 provide a variable voltage divider for the reference voltage of the shunt regulator 126, so the reference voltage varies based on temperature. In a PTC embodiment, the first resistor component 122 is a resistor and the second resistor 124 component is a PTC thermistor. As the temperature increases, the PTC thermistor will increase its resistance at a greater rate than the resistor, which will increase the voltage to the reference input terminal, causing the reference output voltage to drop and causing current to be drawn. Since a PTC can be a device whose resistance changes linearly with respect to temperature, the change in voltage is also substantially linear. As the reference input terminal increases, the threshold voltage (the rating of the reference device) is crossed and the shunt regulator 126 begins to divert (or sink) drive current from the current source (eg, from the dimming interface 110 ) away from the voltage divider part of the voltage, thus reducing the voltage across the positive terminal 129 and the negative terminal 128 of the control circuit 120 . The lower voltage at the dimming interface reduces the current and voltage output of the LED driver 102 (which is determined by the relationship between voltage and current through the diode), which dims the LED. Dimmed LEDs generate less heat and reduce the temperature sensed by control circuit 120 .

In an NTC embodiment, the first resistor component 122 is an NTC thermistor and the second resistor 124 component is a resistor. As the temperature increases, the NTC thermistor will decrease its resistance at a faster rate than the resistor, which will increase the input voltage to the reference terminal, causing the reference output voltage to sink current as the NTC thermistor sinks While down, this is similar to the PTC embodiment. Since an NTC can be a device whose resistance changes linearly with respect to temperature, the change in voltage is also substantially linear. As the reference input terminal increases, the threshold voltage is crossed and the shunt regulator 126 begins to divert (or sink) a portion of the drive current from the current source (eg, from the dimming interface 110 ) away from the voltage divider, which crosses the control circuit Positive terminal 129 and negative terminal 128 of 120 reduce the voltage. The lower voltage at the dimming interface reduces the current and voltage output of the LED driver 102 (which is determined by the relationship between voltage and current through the diode), which dims the LED. PTC or NTC embodiments decrease the reference output voltage (sink more current) as the temperature increases, and increase the reference output voltage (sink less current) as the temperature decreases. When the sensed temperature causes the voltage divider to increase above the threshold voltage (referenced to the rating of the device), current through the shunt regulator 126 is turned on.

Accordingly, the first resistor assembly 122 and the second resistor assembly 124 and the shunt regulator 126 are configured such that as the sensed temperature increases, the resistance of the thermistor changes (eg, increases with PTC or increases with NTC) decrease), which changes the reference voltage input to the shunt regulator 126. When the reference input voltage reaches a certain threshold level (rated value of the reference device), the shunt regulator 126 sinks current and reduces the voltage across the positive terminal 129 and the negative terminal 128 of the control circuit 120 . The lower voltage causes the LED driver 102 to reduce the light output of the LED engine 104 . The threshold level is chosen to be close to the minimum dimming voltage of the 0V to 10V system (typically about 1V) to allow normal operation and to provide dimming control in the presence of excessive heat. In addition to or in place of those described, additional components may be used to form a temperature-sensitive circuit that provides a control signal to a driver to dim or otherwise reduce the light output of the lamp fixture, As will be understood by those skilled in the art upon reviewing the present invention. For example, a potentiometer may be provided to allow the user to adjust the maximum light output of the lamp fixture, or a component may be provided to allow the user to adjust the maximum light output of the light fixture via the thermal foldback device 100 .

FIG. 3 illustrates another embodiment of a thermal foldback control circuit 130 for measuring the temperature at a reference point and providing control signals to the LED driver 102 (FIG. 1). Control circuit 130 includes thermistor RT1 132 (eg, PTC thermistor), resistor R1 134 , shunt regulator IC1 136 (eg, TLV431 ), and capacitor C1 138 implemented with thermistor 132 . The reference terminal Vref of the shunt regulator 136 is electrically connected to the common node of the thermistor 132 , the resistor 134 and the capacitor 138 . Control circuit 130 is connected to LED driver 102 through positive terminal 140 (eg, P1 , purple pin) and negative terminal 142 (eg, P2 , gray pin) of dimmer interface 110 . Thermistor 132, resistor 134, shunt regulator 136, and capacitor 138 provide a PTC embodiment of the thermal foldback control circuit. The control circuit 130 may be powered by a voltage or current supplied from the secondary output voltage from the LED driver 102 . In addition to or in place of those described, additional components may be used to form a temperature sensitive circuit that provides control signals to drivers to dim or otherwise reduce light fixtures light output, as will be understood by those skilled in the art upon reviewing the present invention. The temperature threshold to allow current to flow through the shunt regulator and the amount of current to flow through the shunt regulator are set based on predetermined values of the thermistor 132 , resistor 134 , and shunt regulator 136 .

FIG. 4A shows the relationship between the temperature sensed and the percentage of output current of the LED driver 102 when the thermal foldback device 100 is coupled to the dimming interface 110 of the LED driver 102 . FIG. 4B shows the relationship between temperature and the output voltage of the thermal foldback device 100 for the dimming interface 110 . Temperatures at the reference point that exceed a temperature threshold (eg, but not limited to, about 80°C) activate the thermal foldback mechanism, which reduces the voltage across the dimming interface terminals. Accordingly, the LED driver 102 (FIG. 1) proportionally reduces the current supplied to the light source (eg, the LED module). The current follows a linear line between 100% and the minimum dimmer level (eg, 30% in the depicted embodiment). Light can increase along the same curve as the temperature decreases. If the temperature exceeds another temperature threshold (eg, about 100° C.), the LED driver 102 can completely turn off the light source to protect the lamp fixture. The LED driver 102 may include settings for shutting down the power supply or removing current from the LED when a minimum dimmer level is reached or the thermal foldback device 100 produces a minimum threshold voltage. When the temperature drops to a safe level (eg, a predetermined voltage level (eg, about 80° C.)), the LED driver 102 is turned back on.

According to one embodiment, the thermal foldback device 100 ( FIG. 1 ) is integrated on a single chip or printed circuit board (PCB) 144 as shown in FIGS. 5-8 . The PCB 144 has a relatively small footprint, which allows the thermal foldback device 100 to be mounted to various reference points externally, for example, on the outside of the LED driver housing 146 . The PCB 144 may be mounted on the driver housing 146 at sensitive or hot spots. Hot spots can be determined by analytical calculations or testing (eg, thermal imaging). In the illustrated embodiment, the PCB is mounted to the housing 146 using screws 148, but other mechanical fasteners or adhesive connections may be used. The thermal foldback device 100 is electrically connected to the driver 102 by one or more conductors. In the illustrated embodiment, the conductors are connected to the heat return device by connectors 150 and extend through conduit 152, but only insulated wire conductors may be used.

In some embodiments, the thermal foldback device 100 integrates more than one temperature sensitive unit mounted at different reference points. More than one thermal foldback device 100 may also be positioned at different reference points and connected to the driver 102 . The upper limit of the thermal foldback device 100 and/or the monitored reference point depends on the size and configuration of the associated lamp fixture, as will be understood by those skilled in the art.

FIG. 9 illustrates one embodiment of a method 200 for monitoring and controlling the temperature of a lamp fixture operatively connected to the heat return device 100 . In operation, the thermal refrigerating apparatus 100 detects the temperature at the reference point (block 205). The heat refrigerating device 100 determines whether the detected temperature has exceeded a temperature threshold (block 210). If the detected temperature has exceeded the temperature threshold, then the thermal foldback device 100 reduces the current (block 215 ), with the method 200 then proceeding back to block 205 . If the detected temperature has not exceeded the temperature threshold, then normal operating conditions continue (block 220 ), with the method 200 then proceeding back to block 205 .

10 illustrates an embodiment of a method, operations 300, of controlling a circuit. In operation, as the temperature at the reference point changes, the resistance of the resistor assemblies (eg, resistor assembly 122, resistor assembly 124, thermistor 132, etc.) also changes (block 305). As the resistance of the resistor assembly changes, the voltage of the control circuit will also change (block 310). The voltage of the control circuit is compared to a predetermined voltage of a Zener-type diode or shunt regulator (block 315). It is determined whether the voltage of the control circuit has crossed a predetermined voltage (block 320). If the voltage of the control circuit has crossed the predetermined voltage, the current is reduced, thus dimming the LED (block 325 ), with the method 300 then proceeding back to block 305 . If the voltage of the control circuit has not crossed the predetermined voltage, normal operating conditions continue (block 330 ), with the method 300 then proceeding back to block 305 .

The temperature can be monitored at multiple reference points and the current supplied to the light emitter is reduced when the temperature at any of the reference points crosses a predetermined threshold. The thresholds at each reference point need not be the same, and each threshold can be designed to meet requirements at a particular point of interest. For example, the temperature threshold of the LED driver 102 may be different from the temperature threshold of the LED engine 104 .

In one embodiment, thermal foldback device 100 is physically connected to components of the lamp fixture (eg, driver housing 146 ) and operatively connected through LED driver 102 (eg, through dimmer interface 110 ) to the light-emitting device. In various embodiments, the thermal foldback device 100 is configured to operate at any 0V to 10V control. If a temperature threshold (eg, about 80° C.) is exceeded, the thermal foldback device 100 causes the driver 102 to dim the lighting device, eg, by reducing the current supplied to reduce the brightness and heat output of the lighting device. If the temperature continues to rise, the current supplied to the light emitting device is further reduced. The decrease in current can be linear, curvilinear, or stepped (as desired) with the increase in temperature. A second threshold for completely turning off the light emitting device may also be established.

Accordingly, this application provides text and other content. Various features and advantages of the present application are set forth in the following claims.

Claims (19)

1. A heat foldback control circuit electrically connected to a light emitting diode LED driver, the heat foldback control circuit comprising: voltage divider, which contains a first resistor assembly having a first resistance that varies in response to temperature at a reference point, a second resistor assembly in a series configuration with the first resistor assembly, the second resistor assembly having a second resistance that varies in response to the temperature at the reference point, and an output electrically connected between the first resistor component and the second resistor component and configured to output a reference voltage based on the first resistance and the second resistance, wherein as the the temperature at the reference point increases, the second resistor assembly increases its resistance at a faster rate than the first resistor assembly or the first resistor assembly increases its resistance at a faster rate than the second resistor assembly A component lowers its resistance at a faster rate so that the reference voltage rises; and a shunt regulator in a parallel configuration with the voltage divider, the shunt regulator configured to receiving the reference voltage, and A driver output of the LED driver is controlled based on the reference voltage. 2. The thermal foldback control circuit of claim 1, wherein the driver output powers one or more light emitting diodes (LEDs). 3. The heat foldback control circuit of claim 1, wherein the first resistor assembly is selected from the group consisting of a negative temperature coefficient NTC type thermistor and a positive temperature coefficient PTC type thermistor at least one. 4. The heat foldback control circuit of claim 1, wherein the second resistor assembly is selected from the group consisting of a negative temperature coefficient NTC type thermistor and a positive temperature coefficient PTC type thermistor at least one. 5. The thermal foldback control circuit of claim 1, wherein the shunt regulator comprises at least one selected from the group consisting of a Zener diode, an avalanche breakdown diode, and a voltage regulator transistor. 6. The heat foldback control circuit of claim 1, wherein the shunt regulator reduces drive current in response to the reference voltage crossing a predetermined threshold. 7. The heat foldback control circuit of claim 6, wherein the predetermined threshold is related to a predetermined temperature at the reference point. 8. The thermal foldback control circuit of claim 1, wherein the reference point is located at at least one selected from the group consisting of the LED driver and LED engine. 9. The thermal foldback control circuit of claim 1, further comprising a capacitor in a parallel configuration with the second resistor. 10. A light emitting diode LED system comprising: one or more light-emitting diodes LED; An LED driver that provides power to the one or more LEDs; 3. The heat foldback control circuit of claim 1 electrically connected to the LED driver, the heat foldback control circuit configured to output a control signal to the LED driver based on a temperature at a reference point. 11. The LED system of claim 10, wherein the power provided to the one or more LEDs is based on the control signal. 12. The LED system of claim 10, wherein the thermal foldback control circuit comprises the voltage divider, the voltage divider further comprising an output configured to output a reference voltage based on the first resistance and the second resistance; and a shunt regulator in a parallel configuration with the voltage divider, the shunt regulator configured to receiving the reference voltage, and The control signal is output based on the reference voltage. 13. The LED system of claim 10, wherein the control signal dims the one or more light emitting diodes when the temperature crosses a temperature threshold. 14. The LED system of claim 10, wherein the control signal disables power to the one or more light emitting diodes when the temperature crosses a temperature threshold. 15. The LED system of claim 10, wherein the reference point is located at at least one selected from the group consisting of the LED driver and LED engine. 16. The LED system of claim 10, wherein the LED driver includes a dimmer interface, and the thermal foldback control circuit is electrically connected to the LED driver through the dimmer interface. 17. A method of controlling power to one or more light emitting diodes (LEDs) using the heat foldback control circuit of claim 1, the method comprising: sense the temperature at the reference point; comparing the sensed temperature to a predetermined temperature threshold, and When the sensed temperature crosses the predetermined temperature threshold, power to the one or more LEDs is reduced. 18. The method of claim 17, further comprising When the sensed temperature is below the predetermined temperature threshold, power to the one or more LEDs is restored to a normal level. 19. The method of claim 17, wherein the reference point is located at at least one selected from the group consisting of an LED driver and an LED engine.

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